1. Field of the Invention
This invention relates to processes for depositing compounds by means of chemical vapor deposition and, more particularly, to processes for depositing silicon dioxide layers using ozone and tetraethylorthosilane as precursor compounds.
2. State of the Art
Doped and undoped silicon dioxides, which are commonly referred to as silicate glasses, are widely used as dielectrics in integrated circuits. Although silicon dioxide possesses a tetrahedral matrix which will impart a crystalline structure to the material under proper heating and cooling conditions, the silicon dioxides used as dielectrics in integrated circuits are typically amorphous materials. This application uses the term "silicate glass" to refer to silicon dioxides deposited via chemical vapor deposition (CVD), as the term encompasses materials containing not just silicon dioxide, but dopants and other impurities as well.
Chemical vapor deposition of silicate glasses has become of paramount importance in the manufacture of contemporary integrated circuits. For example, silicate glass doped with both boron and phosphorous is widely used as an interlevel dielectric and as a getter material for mobile sodium ions.
Chemical vapor deposition (CVD) of silicate glasses by the semiconductor industry is most commonly accomplished by reacting tetraethylorthosilane (TEOS), silane or disilane with an oxidizer. Silane is typically reacted with diatomic oxygen (O.sub.2) or nitrous oxide (N.sub.2 O) at a temperature of about 400.degree. C. TEOS, on the other hand, is generally reacted with either O.sub.2 or ozone (O.sub.3). If a low reaction temperature is desirable, the use of ozone permits a reduction in the reaction temperature to about half that required for O.sub.2. For the sake of brevity, glass layers deposited from the reaction of O.sub.3 and TEOS shall be termed "ozone-TEOS glasses". The reaction temperature may also be reduced for the TEOS-O.sub.2 reaction by striking a plasma in the deposition chamber. Glasses deposited via this plasma-enhanced chemical vapor deposition (PECVD) method shall be referred to hereinafter as PECVD-TEOS silicate glasses. The plasma generates highly reactive oxygen radicals which can react with the TEOS molecules and provide rapid deposition rates at much reduced temperatures.
Silane is used for the deposition of silicate glasses when substrate topography is minimal, as the deposited layers are characterized by poor conformality and poor step coverage. Silicate glasses deposited from the reaction of TEOS with O.sub.2 or O.sub.3 are being used with increasing frequency as interlevel dielectrics because the deposited layers demonstrate remarkable conformality that permits the filling of gaps as narrow as 0.25 .mu.m. Unfortunately, the deposition rate of silicate glass formed by the reaction of TEOS and O.sub.3 is highly surface dependent. A particularly acute problem arises when the deposition is performed on a surface having topographical features with non-uniform surface characteristics. For example, the deposition rate is very slow on PECVD-TEOS glass layers, considerably faster on silicon and on aluminum alloys, and faster still on titanium nitride, which is frequently used as an anti-reflective coating for laser reflow of aluminum alloy layers. A correlation seems to exist between the quality and relative deposition rate of ozone TEOS glass layers. For example, ozone TEOS glass layers that are deposited on PECVD-TEOS glass layers have rough, porous surfaces and possess high etch rates.
In U.S. Pat. No. 5,271,972 to K. Kwok et al., it is suggested that the surface sensitivity of ozone-TEOS glass layers deposited on PECVD-TEOS glass layers may be related to the presence of a hydrophilic surface on the PECVD-TEOS glass layers. A hydrophilic surface on the PECVD-TEOS glass layer may be attributable to embedded elemental carbon particles which are formed as the TEOS precursor gas is attached by oxygen radicals generated by the plasma. As elemental carbon particles are characteristically hydrophilic, they repel TEOS molecules, which are characteristically hydrophobic, and interfere with their absorption on the surface of the deposited layer. Thus, the poor absorption rate of TEOS molecules on the surface of PECVD-TEOS glass results in slowly deposited, poor-quality films. Experimental evidence indicates that deposition rates are low for hydrophilic surfaces and high for hydrophobic surfaces. For example, titanium nitride, being highly hydrophobic, readily absorbs TEOS molecules on its surface, which accelerates the deposition reaction.
Given the surface-dependent variation in deposition rates, it is not uncommon for ozone-TEOS glass layers to build up rapidly around aluminum conductor lines and much more slowly on PECVD glass layers on which the conductor lines are fabricated, thereby forming cavities of tear-drop cross section between adjacent conductor lines. FIG. 1 is a cross-sectional view which depicts the undesirable result obtained by conventionally depositing an ozone-TEOS layer 11 over aluminum conductor lines 12 which overlie an underlying PECVD-TEOS glass layer 13. Prior to patterning, the aluminum conductor lines 12 were covered with a titanium nitride layer which served as an anti-reflective coating during a laser reflow operation. A titanium nitride layer remnant 14 is present on the upper surface of each conductor line 12. A cavity 15 having a teardrop-shaped cross section has formed between each pair of conductor lines 12. Cavities in an interlevel dielectric layer are problematic primarily because they can trap moisture when the deposited glass layer is subjected to a planarizing chemical mechanical polishing step during a subsequent fabrication step. The moisture, if not completely removed prior to the deposition of subsequent layers, can corrode metal conductor lines during normal use and operation of the part, or it may cause an encapsulated integrated circuit device to rupture if the steam generated as the device heats up is unable to escape the package.
In U.S. Pat. No. 5,271,972, a technique is disclosed for improving the film quality of ozone-TEOS glass layers deposited on PECVD-TEOS glass layers. The method involves depositing the underlying PECVD-TEOS layer using high pressure and a high ozone-to-TEOS flow rate. For the last several seconds of the plasma-enhanced deposition process, a stepwise reduction in reactor power is carried out. It is claimed that this technique produces an interstitial silicon dioxide layer at the surface of the PECVD-TEOS layer which greatly reduces the surface sensitivity of subsequently deposited ozone-TEOS oxide layers.